The present invention relates to a process for the reductive dimerization of 1,1,1-trifluoro, 2,2,2-trichloroethane (CFC-113a) with hydrogen, which allows to obtain with improved yields 1,1,1,4,4,4-hexafluoro-2,2,3,3-tetrachlorobutane, from now on called SD (saturated dimer), and 1,1,1,4,4,4-hexa-fluoro-2,3-dichlorobutene, from now on called UD (unsaturated dimer), so mixtures thereof.
Said compounds are used in hydrogenation processes known in the prior art for the 1,1,1,4,4,4-hexafluorobutane (HFC-356) synthesis, which is used to replace the CFCs banned by the Montreal Protocol, since it is not dangerous for the ozone layer.
U.S. Pat. No. 5,382,720 describes the production of intermediates to be subjected to hydrogenation to give HFC-356. These compounds are obtained by dimerization in gaseous phase of 1,1,1 trifluoroethane halogenated derivatives containing from one to three chlorine atoms. By this reaction, by using as catalyst nickel, preferably supported on silicon oxide (silica), one or three chlorine atoms of 1,1,1 trifluoroethane can be replaced by fluorine, as shown in the Examples. The use of a silicon oxide-based catalyst gives some problems from the industrial point of view. In the hydrogenation of fluorinated hydrocarbons partially chlorinated with this support, hydrochloric acid is formed together with hydrofluoric acid traces. These acids attack the support itself forming compounds as silicon chlorides and oxyhalides (and fluorides), which at room temperature are generally liquids and some of them also gaseous. Under these conditions the catalyst is degraded. In said patent an additional step is contemplated to absorb on calcium oxide the hydrohalogenic acids present in the reaction gas.
U.S. Pat. No. 5,536,890 relates to a process for the HFC 356 production by dimerization and then by hydrogenation in gaseous phase, on Pd/Ni catalyst on active carbon support, of an halogenated derivative (halaogen=chlorine, bromine) of trifluoroethane, specifically CFC 113a, and subsequent gaseous hydrogenation under the same conditions. The dimerization reaction yields are very low. See the Examples of the patent.
The need was therefore felt to have available an industrial process to produce, starting from CFC 113a, with improved yields, precursors to obtain HFC-356, by using stable catalysts in the reaction conditions.
It has surprisingly and unexpectedly been found by the Applicant a process wherein the dimerization by hydrogenation in vapour phase of FC 113a takes place with improved yields, in the presence of a catalyst supported on the materials described hereinunder.
It is an object of the present invention a process for the reductive dimerization of 1,1,1-trifluoro, 2,2,2-trichloroethane (CFC-113a) with hydrogen, with formation of 1,1,4,4,4-hexafluoro-2,2,3,3-tetrachlorobutane and 1,1,-1,4,4,4-hexafluoro-2,3-dichlorobutene, and mixtures thereof, on a catalyst constituted by metal ruthenium supported on one of the following materials:
a) grafted carbon obtainable by treatment of the carbon at temperatures higher than 2000xc2x0 C., in inert gas, according to known methods in the prior art, having surface area in the range 350-100 m2/g BET, preferably 350-250 m2/g BET, or
b) aluminum fluoride having an high surface area, not lower than 25 m2/g, preferably not lower than 30 m2/g and a pore volume not lower than 0.20 cc/g, prepared by fluorination with gaseous HF of an alumina containing from 0.5 to 15% by weight of silicon oxide, and having a surface area of at least 150 m2/g with pore volume not lower than 0.3 cc/g.
With aluminum fluoride, according to the present invention, the alumina fluorination product is meant, with a fluorine content not lower than 90% by weight, preferably not lower than 95%, with respect to the stoichiometric.
The aluminum fluoride surface area is preferably not lower than 30 m2/g and the pore volume is not lower than 0.25 cc/g.
The particle sizes of the materials used as catalyst support are not critical. The support can be in the form of pellets, having sizes of some millimeters, or of granules with a diameter in the range of some tenths and some hundreds of micron.
The catalyst is prepared by impregnating the support with an aqueous solution of a soluble ruthenium salt, preferably a trivalent ruthenium salt, by using the method known as xe2x80x9cdry impregnationxe2x80x9d. The impregnated support is dried in stove at 110xc2x0 C. for some hours, then, preferably, calcined at 400xc2x0 C. in air flow for 4 hours, reducing at the end the ruthenium cation to metal by treatment with hydrogen at 400xc2x0 C. for 7-8 hours.
It has been found by the Applicant that in the absence of the calcination step with air the 113a conversion is higher, but it quickly decreases in the time, while the selectivity is lower (see the Examples).
Preferably the trivalent ruthenium salt is ruthenium chloride RuCl3.
Generally the metal reduction treatment is carried out inside the same reactor wherein the 113a dimerization reaction takes place.
The catalyst ruthenium content is in the range 1-10% by weight, preferably 3-5%.
The reaction is carried out by feeding in the reactor 113a in gaseous phase, pure or optionally in admixture with an inert gas, for example helium.
The 113a feeding to the reactor, determined as ratio between the 113a weight and that of the catalyst, is in the range 0.1-10 hxe2x88x921 (WHSV: weight hourly space velocity).
The reaction temperature is in the range 50xc2x0-250xc2x0 C., preferably 100xc2x0-200xc2x0 C. At higher temperatures the conversion increases but the selectivity decreases. At temperatures lower than 50xc2x0 C. the conversion is insufficient.
Preferably the hydrogen/113a molar ratio is in the range 3-10.
The pressure is not critical and preferably one operates at atmospheric pressure.
During the use carbonaceous residues deposit on the catalyst, which gradually decrease the efficiency thereof. The catalyst can be easily regenerated by oxidation in air flow at 300xc2x0-400xc2x0 C. for 1 h and subsequently treating with hydrogen at 400xc2x0 C. for 1.5 h.
The preparation of the aluminum fluoride having a high surface area mentioned in b) is described in the European patent application No. 879,790 in the name of the Applicant herein incorporated by reference.
For the fluorination conditions of the alumina containing silica, one generally operates at temperatures in the range of about 250xc2x0-450xc2x0 C., preferably 300xc2x0-400xc2x0 C. One operates at atmospheric pressure, in the presence of an inert gas in the reaction conditions, for example air or nitrogen; the HF partial pressure must be in the range 0.1-0.5 Atm.
Preferably the reaction takes place in a fluidized bed process. It has been found that in this way the optimal control of the reaction temperature is obtained. For applying this reaction technique the alumina to be fluorinated must have a particle size compatible with the use of fluidized beds.
Generally the alumina used according to the present invention contains less than 0.1% by weight of each of the contaminants of AlF3 final product, such as: iron, sulphates, sodium.
When the aluminas are in the hydrated form, before fluorination it is preferable to calcine in air or in nitrogen atmosphere, at temperatures in the range 300xc2x0-400xc2x0 C. This limits the water development during the reaction, undesirable since it favours the equipment corrosion.
The aluminas containing silica are prepared by known methods in the prior art, for example spray-drying of suitable precursors, and they are commercial products, e.g. those of the firm Condea Chemie (Germany).
The physical characterization of the aluminas and of the aluminum fluorides is carried out by using techniques well known to the skilled man in this field:
the surface area, determined by nitrogen adsorption according to the BET method;
the pore volume, measured by mercury intrusion at high pressure;
the crystalline phases by X ray diffraction;
the constituent analyses are carried out by wet way according to known methods, or by X ray fluorescence by comparison with standards prepared on the same matrix through calibrated additions.
The obtained AlF3 generally consists prevailingly of gamma phase as described in FR 1,383,927.
The support indicated in a) is commercially available, for example TIMREX(copyright) produced by the firm TIMCAL of (CH 5643) Sinsxe2x80x94Switzerland.
The Examples reported hereinunder have the aim to illustrate, without anyway limiting it, the process object of the present invention.